A method for producing silver halide grains comprises two main processes of nucleation and grain growth. There are disclosed in T. H. James, The Theory of the Photographic Process, 4th Ed., Macmillan (1977) that "Nucleation is a process in which new crystals are formed, wherein rapid increase of the number of crystals occurs. Growth means the addition of new layers to already existing crystals. In addition to the above nucleation and grain growth, further two more processes of Ostwald ripening and recrystallization occur under the certain conditions of the grain formation of photographic emulsion grains. Ostwald ripening is liable to come about when a grain size distribution is wide under comparatively high temperature and in the presence of a silver halide solvent. Recrystallization is the process of changing of crystal structure." That is, a grain nucleus is formed at early stage of the silver halide grain formation and the growth of grains is conducted only by already existing nuclei and the number of grains during growing process does not increase.
Silver halide grains are in general produced by the reaction of a silver salt aqueous solution and a halide aqueous solution in a colloid aqueous solution in a reaction vessel. A single jet method in which protective colloid, such as gelatin, and a halide aqueous solution are added into a reaction vessel and, while vigorously stirring the protective colloid and the halide aqueous solution, a silver salt aqueous solution is added thereto over a certain period of time; and a double jet method in which a gelatin aqueous solution is added into a reaction vessel and a silver salt aqueous solution and a halide aqueous solution are added thereto over a certain period of time at the same time are known. When two methods are compared, silver halide grains having a narrow grain size distribution can be obtained and the halide structure can be freely changed with the progress of the grain growth according to a double jet method.
It has been known that the nucleation and the growth of silver halide grains are largely varied according to the silver ion (halide ion) concentration, the concentration of a silver halide solvent, the degree of supersaturation and the temperature of a reaction solution. In particular, nonuniformity of the concentration of silver ion or halide ion produced by a silver salt aqueous solution and a halide aqueous solution added to a reaction vessel causes uneven degrees of supersaturation and solubility in a reaction vessel due to each uneven distribution. As a result, the nucleation speed or grain growing speed becomes uneven in the reaction vessel leading to nonuniformity of silver halide crystals produced.
For lowering this nonuniformity, rapid and uniform mixing and reaction of a silver salt aqueous solution and a halide aqueous solution supplied to a colloid solution are necessary to make uniform concentration of a silver ion or a halide ion in a reaction vessel during silver halide grain formation. Various studies have been so far made as to the realization of the uniform mixing of a silver salt aqueous solution and a halide aqueous solution, for example, contrivances of stirring and mixing apparatuses for solving the above problem are disclosed in U.S. Pat. No. 3,415,650, British Patent 1,323,464, U.S. Pat. No. 3,692,283, JP-B-55-10545 (the term "JP-B" as used herein means an "examined Japanese patent publication"), and JP-A-57-92523 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Producing methods and mixing apparatuses disclosed in these patents consist of a structure comprising a casing having an open area and a rotor in the casing which is provided in the reaction vessel, and a silver salt aqueous solution and a halide aqueous solution are added to a mixing vessel and both are rapidly mixed while diluting with a colloid solution in the reaction vessel.
However, although local nonuniform concentrations of a silver ion and a halide ion in the reaction vessel of these apparatuses can be certainly solved, nonuniform concentrations still exist in the mixing vessel, in particular, a considerable uneven concentrations are distributed in the vicinity of a feeding nozzle of a silver salt aqueous solution and a halide aqueous solution and around stirring blades. Silver halide grains supplied to the mixing vessel with protective colloid pass through such nonuniform part and a more important fact is that silver halide grains rapidly grow at these parts. This means grain growth occurs at nonuniform parts of concentration. Therefore, uniform silver halide grain growth cannot be attained.
Producing methods in which a reaction vessel is independent of a mixing vessel for solving the distribution of nonuniform concentrations of a silver ion and a halide ion by more complete mixture are disclosed in JP-A-53-37414, JP-B-48-21045 and U.S. Pat. No. 3,897,935. However, the colloid aqueous solution in the reaction vessel of these apparatuses is also circulated to the mixing vessel and silver halide grains also grow with passing through nonuniform parts.
Producing method for solving these problems are disclosed in JP-B-7-82208, JP-B-7-23218 and U.S. Pat. No. 4,879,208. According to these methods, a mixing vessel is provided at the outside of a reaction vessel where nucleation and/or grain growth of silver halide grains are caused, a silver salt aqueous solution and a halide aqueous solution are supplied to said mixing vessel and mixed therein to form silver halide fine grains, the formed fine grains are immediately supplied to a reaction vessel and nucleation and/or grain growth are conducted within said reaction vessel. This method is characterized in that silver halide formation by the addition of a silver salt aqueous solution and a halide aqueous solution is substantially not conducted in the reaction vessel where nucleation and/or grain growth of silver halide grains are carried out, and further the circulation of the emulsion in the reaction vessel to the mixing vessel is not conducted at all. According to this method, extremely fine grains formed in the mixing vessel are introduced into the reaction vessel and then dispersed in the reaction vessel by stirring. As grain sizes are extremely fine, grains are easily dissolved and silver ions easily release halide ions. As a result, uniform nucleation and/or grain growth can be caused.
A method for producing an extremely thin tabular grain emulsion having an average thickness of less than 0.07 .mu.m according to Dual Zone Process is disclosed in European Patent Application No. 507701A. This patent discloses a method for producing extremely thin tabular grains by the same methods as disclosed in the above JP-B-7-23218, JP-B-7-82208, U.S. Pat. No. 4,879,208 and European Patent Application No. 326852B. According to this method, tabular grain nuclei are formed by adding a silver nitrate aqueous solution, an NaBr aqueous solution and a gelatin aqueous solution respectively independently to a mixing vessel within a short period of time (from 0.5 minutes to 2 minutes), the silver halide fine grain emulsion formed is transferred as it is to a reaction vessel containing a gelatin aqueous solution, ripened, thus tabular grain nuclei are formed, thereafter silver halide fine grains are further transferred in the same manner from the mixing vessel to the reaction vessel and tabular grain nuclei are grown to obtain extremely thin tabular grain emulsion.
It has become possible to produce thin tabular grains having such the high uniformity but an important point in this method is to form extremely fine grains having a finer grain size well controlled in a mixing vessel.
The mixing vessel shown in FIG. 5 is disclosed in JP-B-7-82208, JP-B-7-23218 and U.S. Pat. No. 4,879,208. In FIG. 5, 7 indicates a mixing apparatus, a reaction vessel 1 is provided therein, and a rotary shaft 11 fitted with stirring blades 9 is provided within the reaction vessel 1. A silver salt aqueous solution, a halide aqueous solution and a protective colloid aqueous solution are supplied from three feeding ports (4 and 5, another is omitted from the figure), mixed rapidly and vigorously by rotating the rotary shaft at high speed (1,000 rpm or more, preferably 2,000 or more, more preferably 3,000 rpm or more), the solution containing extremely fine silver halide grains formed is immediately discharged through a discharging port 8 to the outside and added to a reaction vessel where nucleation and/or grain growth of silver halide grains are conducted.
A concrete example of this mixing machine is shown in FIG. 6. This apparatus is one manufactured by WILLY A PACHOFEN AG MASHINEN FABRIK. This apparatus is constructed of a stirring tank 42 of an almost cylindrical shape and a plurality of stirring blades 43 which are rotation-driven in the stirring tank 42. The stirring tank 42 is an almost closed vessel of the structure provided with a solution-feeding port 44 on one side to introduce an objective solution of stirring and a solution discharging port 45 on the other side to discharge the solution after stirring processing. A plurality of stirring blades 43 are fixed on a sleeve laid on a rotary shaft 46 protruding through the end wall of the other side of the stirring tank 42. These stirring blades rotate in a body with the rotary shaft 46 through the sleeve to accelerate stirring of the solution in the stirring tank 42. The rotary shaft 46 is rotation driven by the motor shown in the figure.
There are problems in the mixing vessel shown in FIG. 5 with respect to the following two points.
1) As shown in FIG. 5, as the rotary shaft 11 protrudes through the mixing apparatus 7, sealing is necessary at the protruding part. In particular, for forming homogeneous and extremely fine grains having finer grain size in the mixing vessel, stirring blades 9 should be rotated at high speed but sealing is liable to become incomplete due to this high speed rotation. Therefore, the high speed rotation mixing could not be realized. As the sealed part is required not only to prevent the leakage of the solution within the tank but also to have lubricating capability, liquid seal is sometimes employed but it is very difficult to maintain liquid seal in an ideal condition and in some cases a problem such that the sealing liquid used for the liquid seal is mixed in the solution in the stirring tank as impurities arises.
2) In FIG. 5, when stirring blades 9 are rotated at high speed (2,000 rpm or more), strong centrifugal force operates within the vessel and the solution in a mixing space 10 is pushed to the wall of the mixing vessel, as a result, a cavity is generated at the central part. Therefore, mixing of the solutions added cannot be conducted efficiently and the improvement of mixing capability by the increase of the rotating speed of stirring blades can not be obtained, and further mixing capability is sometimes reduced.
Further, in the conventional structure shown in FIG. 6, at the part of the end wall of the stirring tank 42 where the rotary shaft 46 protrudes, not only sealing capability to prevent the leakage of the solution which has been stirred and mixed to the outside but also lubricating capability necessary for smooth and high speed rotation of the rotary shaft 46 are required. For satisfying these both requirements, in general, liquid seal is employed as a sealing means but it is very difficult to maintain liquid seal in an ideal condition and in some cases the lubricating liquid (sealing liquid) used for liquid seal is mixed in the stirring tank 42 as impurities and impairs emulsifying capability. Further, when the solution mixed with the lubricating liquid is stirred, the removal of the lubricating liquid from the solution is very difficult.
Moreover, as the rotation of respective stirring blades 43 is in the same direction, the flow of the solution in the tank is liable to be regularized, and when the rotating speed of the rotary shaft 46 is increased for improving stirring efficiency, a cavity is generated around the sleeve 47, which is the center of the stirring tank 42, and the solution to be stirred and mixed flows in the tank along the inner peripheral surface of the stirring tank 42 then exhausted, as a result, a silver salt aqueous solution and a halide aqueous solution added are discharged without being mixed sufficiently.